1. Scope of the Invention
The present invention relates to a power recovery circuit for impact printers and more particularly for dot matrix impact printers.
2. Description of Prior Art
It is known that in dot matrix impact printers, printing is performed by selectively energizing printing elements, each comprising a magnetic circuit and a winding magnetically coupled to the circuit, for magnetizing or demagnetizing it, by means of to a suitable energization current flowing in the winding.
In order to obtain high printing performances, the windings must be energized and deenergized as fast as possible.
Several arrangements have been proposed to this purpose. U.S. Pat. No. 3,909,681, describes a printing electromagnet driving circuit in which the current flowing in the electromagnet winding is controlled by a first switch upstream of the winding and a second switch located downstream of the winding.
Two diodes, normally reverse biased, provide a recycling path for current flowing in the winding. The recycling path comprises the voltage source used to energize the winding.
When the two switches are closed a current flows in the winding due to the voltage source. When one of the switches is opened, the current may flow in the winding through the closed switch and one of the diodes. When both the switches are opened the current flows in the winding through both diodes and the voltage source which opposes the current flow.
The current quickly decays and the winding is deenergized rapidly by transferring the magnetic energy to the same voltage source which provided the energization.
The circuit is very effective, has a high efficiency but requires the use of two switches, two related control circuits and two diodes. It is, therefore, that each printing element requires its own driving circuit.
Alternative arrangements have been proposed which are shown in U.S. Pat. No. 316.056. In such arrangements a single switch is used to control each winding. Each winding is provided with a current recirculation path, normally reverse biased and comprising a resistor and a zener diode. In this circuit, the magnetic energy of the winding is transferred to and wasted in the resistance of the recirculation path.
Power losses in electronic circuits have severe implications which may be summarized in:
increased size and cost of the power supply.
heating of the equipment and need for efficient heat dissipators such as fans, which imply a further cost and size increase of the equipment.
Therefore the simplification in the driving circuits for the printing elements is achieved with the trade off due to the increased cost of other elements and to a reduced efficiency of the equipment.
The present invention overcomes such disadvantages and provides a power recovery circuit which, when added to a conventional power supply for the electronic equipment, allows the use of very simple driving circuits for the printing elements and to obtain from such circuits both a fast deenergization of the electromagnet windings as well as a substantially complete recovery of the magnetization energy.